1. Field of the Invention
The present invention relate to a polymerizable liquid crystal compound, a polymerizable liquid crystal composition including the compound and a polymer formed by the polymerization of the compound or the composition.
2. Related Art
A polymerizable liquid crystal compound has recently been utilized for a polymer having optical anisotropy such as a polarizing plate and an optical retardation plate. This is because the compound exhibits optical anisotropy in the liquid crystal state and then the oriented state is fixed by polymerization. The kinds of optical characteristics required for a polymer having optical anisotropy are different depending on a purpose for use, and a compound having characteristics that suit the purpose is necessary. In the compound used for such a purpose, characteristics regarding its polymer are important in addition to the characteristic of optical anisotropy. Characteristics required for the polymer include the rate of polymerization, transparency, mechanical strength, applicability, solubility, the degree of crystallinity, shrinkage, water permeability, water absorptivity, melting points, glass transition temperature, clearing points, chemical resistance and thermal resistance.
A compound having an acryloyloxy group as a polymerizable group among polymerizable liquid crystal compounds has been widely used for such purposes (patent documents Nos. 1 and 2). The acrylate has a high reactivity and the polymer derived from it has a high transparency. However, it is necessary to carry out the reaction in an atmosphere of an inert gas and to increase energy of ultraviolet irradiation, since the type of the reaction is radical polymerization. Thus, an improvement of workability such as curing in air has been required. On the other hand, a compound having an oxiranyl group is also utilized for such purposes (non-patent documents Nos. 1 and 2). The oxiranyl compound is excellent in curability in air and in workability for the curing, since cationic polymerization is not inhibited by oxygen. However, the oxiranyl compound is not preferable to the acrylate in its reactivity. The polymers of these compounds have been required to be improved in characteristics such as heat resistance, shrinkage, adhesive properties, adhesion and mechanical strength in addition to the characteristics described above. The polymers is used as ink in which an organic solvent is added to the polymers for the purpose of adjusting applicability. A polymerizable liquid crystal compound, a photopolymerization initiator, a surfactant and so forth are dissolved in an organic solvent, forming the ink, in which the viscosity of the solution and the leveling properties and so forth are adjusted, for the production of a film having optical anisotropy (which is expressed as an optically anisotropy film in this invention). The ink is applied to a transparent substrate that has been aligned, the solvent is dried, and then the polymerizable liquid crystal compound is oriented. Next, the compound is polymerized with ultraviolet irradiation or by heating, and thus the oriented state is fixed. An organic solvent such as propylene glycol monoethyl ether acetate (PGMEA) is desirable for a solvent used in this procedure in view of environmental load and safety (mutagenicity and toxicity). However, the compounds disclosed in non-patent documents Nos. 1 and 2 have not a high solubility in such a solvent for preparing the ink with a suitable concentration.
3. Prior Art
Conventional compounds are disclosed in the following patent documents: No. 1, JP H07-017910 A (1995); and No. 2, JP H09-316032 A (1997).
Conventional compounds are disclosed in the following non-patent documents: No. 1, Polymer Chemistry, 1993, 31 (9), 2249-60; and No. 2, Polymer, 1994, 35 (3), 622-9.